1. Field of the Invention
The present invention is generally directed to a process of making stain resistant nylon fibers, and the process of making and the composition of a highly sulfonated nylon copolymer used in the making of the stain resistant nylon fibers.
2. Background
Nylon fiber is widely used in the manufacture of carpets and other textile materials. For aesthetic and utilitarian purposes, nylon fiber may be colored using a variety of techniques. One method is to dye the fiber using acid dyes which are typically synthetic and natural anionic dyes with relatively low molecular weights that carry from one to three sulfonic acid groups. Acid dyes are known to ionically bond to the terminal amino, imido and other positively charged site in the nylon polymer.
Another way to color nylon fiber is to include a pigment in the polymer melt from which the nylon fiber is spun. By using pigments to impart color to the nylon fiber, a permanent and more uniform coloration of the fiber may be achieved. Unfortunately, pigmented nylon fiber can be unintentionally "dyed", that is to say stained, by natural or artificial acid dyes existing in some foods and drinks when they are spilled onto nylon carpets and other textile materials.
A common solution to this problem is to topically apply to the surface of the pigmented nylon fiber a material that will function as a "stain-blocking" agent. Although very effective in the short term, such treatments wear off long before the end of the useful life of the carpet or textile material. Further, such treatment may be accidentally removed by detergents or other cleaning agents that may be used on the carpet or other textile material.
Alternatively, it is known that small amounts of certain materials that impart cationic dyeability to nylon, such as aromatic sulfonates and their alkali metal salts, can be copolymerized with the nylon as a means of rendering the nylon resistant to staining by synthetic and natural acid dyes. For example in U.S. Pat. No. 5,108,684 a method of making an acid dye resistant, pigmented nylon fiber is disclosed. The stain resistant nylon fiber is made from a sulfonated nylon polymer containing up to 4% aromatic sulfonate. The sulfonated polymer is made by the inclusion of up to 4% aromatic sulfonate or alkali metal salts in the nylon polymerization reaction. Specifically the sodium salt of 5-sulfoisophthalic acid is taught as being useful. Further improvements on this technology are disclosed in a number of U.S. patents in which the above fiber is treated with various chemical agents after being spun to further enhance the stain resistance of the fiber. One disadvantage of this technology is that in order to "custom formulate " the level of sulfonation, a custom batch of polymer must be formulated to meet the desired sulfonation level.
Another approach to achieving stain resistant nylon fiber is to melt blend regular nylon with a sulfonated nylon polymer that has sulfonated aromatic groups as part of the polymer structure. U.S. Pat. No. 3,846,507 teaches that a highly sulfonated random terpolymer, made by the polymerization reaction of isophthalic or terphthalic acid, a salt of an aromatic sulfonate, and hexamethylene diamine, may be blended with regular nylon to give a smooth acid dye resistant fiber. Particularly, the sulfonated nylon terpolymer resulting from the polymerization of a mixture of isophthalic acid, the sodium salt of 5-sulfoisophthalic acid and hexamethylene diamine is taught as being useful. It is taught that the inclusion of the isophthalic or terphthalic acid is critical if the sulfonated nylon polymer is to be useful in the melt blending and spinning of commercially useful fibers. When the isophthalic or terphthalic acid is not included in the formulation of the highly sulfonated nylon polymer, the resulting blended nylon gives a fiber that is beady and uneven. It is well known in the art that fibers that are beady and uneven have poor appearance and break easily and therefore cannot be used in the formation of carpets or other textile materials.